Infrared optical systems are widely used in a variety of applications, such as imaging, defense, homeland security, law enforcement, airborne intelligence, surveillance, reconnaissance, biological and physiological sensing, spaceborne applications, robotics, astronomy, etc. Depending on the application of the optical system, the spectral band pass, packaging, cost and performance may be based on a single lens system, or a complex multi-element optical system. The basic configuration of an infrared optical system may be a telescopic or a focal system, or may be a focal system forming an image of a scene at a detector plane. These IR optical systems may also be configured as non re-imaging or re-imaging, depending on packaging requirements and operational requirements.
The infrared lenses of the system may be spectral band pass limited due to the transmission and absorption properties of the lens materials and its operational environment. With respect to the spectral band pass, infrared optical systems are commonly used in the short wave infrared range (SWIR, 0.7 to 3.0 microns band), the mid wave infrared range (MWIR, 3.0 to 5.0 microns band), and the long wave infrared range (LWIR, 6 to 12 microns band). Refractive optical materials may be selected for the optical lenses depending on the amount of transmission desired in the band of operation.
An image produced by an optical system has imperfections, called aberrations. These aberrations may take many forms. For example, chromatic or color aberration is a consequence of dispersion. Shorter wavelengths are bent the most and, consequently, focus nearest to the lens; while the longer wavelengths are bent the least and, consequently, focus farther from the lens. Spherical aberration results when spherical lens surfaces are used. Light striking nearer to the periphery of the spherical lens focuses closer to the lens, while light striking near the center of the lens focuses farther away from the lens.
In addition, an optical system is generally affected by changes in temperature. The effective focal length and the back focal distance of an optical system, depending on the materials of the lenses, may increase or decrease as temperature rises. As temperature varies, the image plane of the sensor moves because of changes in the index of refraction and the radii of the lenses.
Conventionally, athermalization and achromatization of optical systems were generally corrected by mechanical devices, usually resulting in compromised image quality. In U.S. Pat. Nos. 5,978,132, issued on Nov. 2, 1999 and 5,909,308, issued on Jun. 1, 1999, to the same inventor Wilhem Ulrich, optical systems are disclosed that correct both, chromaticity and focal shift over thermal variations. Both of these patents are incorporated herein by reference.
More specifically, the patents disclosed by Ulrich provide re-imaging optical systems that are achromatic in the 7.5-10.5 microns band pass range. Each re-imaging optical system includes an athermalized lens system having a front objective, an intermediate image plane, and a relay optic limited to the infrared range of about 10 microns.
In addition, these re-imaging systems utilize two types of lens materials for achromatization and athermalization in the 7.5-10.5 microns wave band. The lens materials include ZnSe or ZnS negative lenses and positive lenses made of special chalcogenide glass IG6 or IG4 type, from Vitron Spezialwerkstoff of Jena, Germany. These lenses are expensive and not readily available.
As will be explained, the present invention includes a re-imaging system that is both achromatic and athermalized, uses lenses made from readily available materials, has a focal length much greater than the short focal length disclosed by Ulrich, and operates in the 3 to 5 micrometer waveband range.